1. Field of the Invention
The present invention relates to an image sensor chip, an image sensor using the same, and methods of driving them and, more particularly, to an image sensor chip for reading an image by photoelectric conversion, an image sensor having a plurality of arrayed image sensor chips, an image sensor chip driving method used therefor, and an image sensor driving method.
2. Related Background Art
Conventionally, a contact-type image sensor (linear image sensor) constituted by arraying light-receiving elements such as photodiodes on a line to directly read image information of an original or the like is often used in an image reading apparatus such as a facsimile apparatus, a digital copying machine, or a scanner. A plurality of image sensor chips serially arrayed in the contact-type image sensor are formed from silicon wafers. For this reason, the sensor length is limited by the wafer size, so an image sensor chip having the same length as the read original width can hardly be manufactured. Hence, conventionally, an imaging optical system is used to form a reduced image of a read original and read the image of the original. However, an apparatus using the reduction imaging optical system requires a space for the optical system and can hardly be made compact. In addition, it is difficult to obtain a sufficient resolution.
To solve the above problems, a so-called contact-type image sensor formed by linearly arraying a plurality of image sensor chips is popularly used. Conventional contact-type image sensors are disclosed in, e.g., Japanese Laid-Open Patent Application No. 2-210947, 2-210948, 2-210949, 2-210950, or xe2x80x9cHigh-Speed, High-Gradation Contact-type Linear Sensor (BASIS Type) Multi-chip Contact Sensorxe2x80x9d, Journal of Television Society, Vol. 16, No. 58, Sep. 25, 1992.
FIG. 1 is a schematic view showing a conventional image sensor.
Referring to FIG. 1, each of image sensor chips 1-1 to 1-m has n light-receiving elements 2-1-1 to 2-1-n arrayed on a line in the x direction. The chip 1-1 also has a selection circuit 3-1 for sequentially selecting the outputs from the light-receiving elements 2-1-1 to 2-1-n, i.e., reading light signals by the light-receiving elements. The chip 1-1 also has a delay circuit 11-1 for delaying the start signal for the read operation of the light-receiving elements. The chip 1-1 also has a clamp circuit 4-1 for clamping the output signal line of the selection circuit 3-1 to a reference potential Vref after the start signal is input and until the selection circuit 3-1 starts the light signal read operation by the light-receiving elements 2-1-1 to 2-1-n. The image sensor also has an amplification circuit 12-1 which is operated by the start signal to amplify the output from the clamp circuit 4-1. The output from the amplification circuit is output from the chip through an output signal line 5-1.
The delay circuit 11-1 is driven by an external driving circuit through an input signal line 8-1.
A control circuit 6-1 for outputting an ON/OFF switch control signal to the clamp circuit 4-1 is driven by an external driving circuit through the input signal line 8-1.
A control circuit 13-1 for outputting an ON/OFF switch control signal to the amplification circuit 12-1 is driven by an external driving circuit through the input signal line 8-1.
The reference voltage Vref is supplied from an external bias circuit through a reference voltage input line 10-1.
The reference voltage input lines 10-1 to 10-m are short-circuited outside the chips, so the common reference voltage Vref is supplied to all chips.
Each of the image sensor chips 1-2 to 1-m has the same arrangement as that of the image sensor chip 1-1, and the same reference numerals as in the image sensor chip 1-1 denote the same parts in these image sensor chips.
The image sensor chips 1-1 to 1-m are arrayed on a line in the x direction, so a total of nxc3x97m light-receiving elements are arrayed on a line in the x direction to form a contact-type image sensor. In reading an image, an original holding the read image is conveyed in the y direction relative to the contact-type image sensor while making the original correspond to the light-receiving elements in units of lines.
The outputs from all light-receiving elements of the image sensor chip 1-1 are sequentially selected by the selection circuit 3-1, and externally output from the output signal line 5-1 through the clamp circuit 4-1 and amplification circuit 12-1. Subsequently, the outputs from all light-receiving elements of the chip 1-2 are sequentially selected by a selection circuit 3-2 and externally output from an output signal line 5-2. In this manner, outputs from all light-receiving elements are time-serially output.
In the conventional so-called contact-type image sensor formed by arraying a plurality of image sensor chips on a line, random noise is generated in the section from each light-receiving element to the clamp circuit of each chip, and a variation in clamp level in units of linearly arrayed image sensor chips appears as shade on the image.
It is an object of the present invention to provide an image sensor capable of obtaining a high image quality and an image sensor chip for forming the image sensor.
It is another object of the present invention to provide an image sensor for reducing random noise in an image sensor chip of the image sensor and reducing shade on an image due to a variation in clamp level in units of linearly arrayed image sensor chips, and an image sensor chip for forming the image sensor.
It is still another object of the present invention to provide an image sensor or image sensor chip driving method optimum to obtain a high image quality.
In order to achieve the above objects, according to an aspect of the present invention, there is provided an image sensor chip used in an image sensor formed by arraying a plurality of image sensor chips each having a plurality of light-receiving elements, comprising a noise reduction circuit for reducing random noise contained in an output signal from the light-receiving element in an ON state and outputting the random noise contained in the output signal from the light-receiving element without any processing in an OFF state, a first control circuit for controlling the ON/OFF state of the noise reduction circuit, a clamp circuit for clamping an output signal output through the noise reduction circuit to a reference potential, and a second control circuit for controlling an ON/OFF state of the clamp circuit.
According to another aspect, there is provided an image sensor formed by arraying a plurality of image sensor chips each having a plurality of light-receiving elements, comprising a noise reduction circuit for reducing random noise contained in an output signal from the light-receiving element in an ON state and outputting the random noise contained in the output signal from the light-receiving element without any processing in an OFF state, a first control circuit for controlling the ON/OFF state of the noise reduction circuit, a clamp circuit for clamping an output signal output through the noise reduction circuit to a reference potential, and a second control circuit for controlling an ON/OFF state of the clamp circuit.
According to still another aspect, there is provided a method of driving an image sensor chip having a plurality of light-receiving elements comprising a noise reduction circuit for reducing random noise contained in an output signal from the light-receiving element in an ON state and outputting the random noise contained in the output signal from the light-receiving element without any processing in an OFF state,
a first control circuit for controlling the ON/OFF state of the noise reduction circuit,
a clamp circuit for clamping an output signal output through the noise reduction circuit to a reference potential, and
a second control circuit for controlling an ON/OFF state of the clamp circuit,
comprising the steps of:
setting the noise reduction circuit in the ON state during only a period except a period when the light-receiving elements read a light signal and setting the noise reduction circuit in the OFF state during the period when the light-receiving elements read the light signal; and
setting the clamp circuit in the ON state during only an ON period of the noise reduction circuit and setting the clamp circuit in the OFF state during a period except the ON period of the noise reduction circuit.
According to still another aspect, there is provided a method of driving an image sensor formed by arraying a plurality of image sensor chips each having a plurality of light-receiving elements,
the image sensor comprising
a noise reduction circuit for reducing random noise contained in an output signal from the light-receiving element in an ON state and outputting the random noise contained in the output signal from the light-receiving element without any processing in an OFF state,
a first control circuit for controlling the ON/OFF state of the noise reduction circuit,
a clamp circuit for clamping an output signal output through the noise reduction circuit to a reference potential, and
a second control circuit for controlling an ON/OFF state of the clamp circuit,
comprising the steps of:
setting the noise reduction circuit in the ON state during only a period except a period when the light-receiving elements read a light signal and setting the noise reduction circuit in the OFF state during the period when the light-receiving elements read the light signal; and
setting the clamp circuit in the ON state during only an ON period of the noise reduction circuit and setting the clamp circuit in the OFF state during a period except the ON period of the noise reduction circuit.
According to still another aspect, there is provided an image sensor formed by arraying a plurality of image sensor chips each having a plurality of light-receiving elements, comprising:
a noise reduction circuit for reducing random noise contained in an output signal from the light-receiving element using a low-pass filter in an ON state and passing through the random noise contained in the output signal from the light-receiving element without any processing in an OFF state;
first switch means for switching between the ON state and the OFF state;
a clamp circuit for clamping an output signal output through the noise reduction circuit to a reference potential common to the plurality of image sensor chips; and
second switch means for switching between an ON state wherein the reference voltage is supplied to the clamp circuit and an OFF state wherein a supply path of the reference voltage is cut.
According to still another aspect, there is provided an image sensor chip comprising:
a plurality of light-receiving elements;
a noise reduction circuit for reducing random noise contained in an output signal from the light-receiving element; and
a clamp circuit for clamping a signal to a predetermined reference potential,
wherein the output signal from the light-receiving element has an input path leading to the clamp circuit through the reduction circuit and an input path leading to the clamp circuit without the mediacy of the reduction circuit.
According to still another aspect, there is provided an image input apparatus comprising:
(A) an image sensor formed by arraying a plurality of image sensor chips,
each image sensor chip including:
(a) a plurality of light-receiving elements:
(b) a noise reduction circuit for reducing random noise contained in an output signal from the light-receiving element; and
(c) a clamp circuit for clamping a signal to a predetermined reference potential,
wherein the output signal from the light-receiving element has an input path leading to the clamp circuit through the reduction circuit and an input path leading to the clamp circuit without the mediacy of the reduction circuit;
(B) a light source for irradiating an object with light; and
(C) a lens array for focusing the light reflected by the object and forming an image of light on the plurality of light-receiving elements in the image sensor.
Other objects, features, and advantages of the present invention will be apparent from the following specification and accompanying drawings.